Rob, assuming you are in England or Wales, the common law position is there is no obligation to fence land what so ever.
It is the responsibility of animal owners to fence against their stock.
So in your case, if you have no animals in your land, where your land borders the farmers he needs to contain his stock.
If you too have stock it's a good idea to get together and share costs of a fence, but there is no obligation to do so.
On the road side you need do nothing, but you would possibly want to do something to make trespass not too easy. There are no particular rules I am aware of other than planning...which i think means max height one metre. And I guess common sence stuff like not to put razor wire next to a kids playground !
In Scotland the law is different.